The invention relates to a process for producing fluoride crystals, in particular calcium fluoride crystals, having high radiation resistance to ultraviolet radiation, which comprises: provisioning of a crystal powder containing alkali metal fluoride or alkaline earth metal fluoride to form a raw crystal mass, melting of the raw crystal mass in a crystal growing unit and solidifying of the molten raw crystal mass by cooling. The invention also relates to a fluoride crystal produced by this process and also an optical component produced from such a fluoride crystal.
Large format, highly homogeneous and ideally scattering-free single crystals (or ingots) of alkali metal fluorides or alkaline earth metal fluorides, in particular CaF2, are typically produced in a one-stage or multistage process from a raw crystal mass in a growing furnace. Such crystals, in particular calcium fluoride crystals, have a high transmission at wavelengths in the (near) UV range, e.g. at 193 nm. These can therefore be used as transmissive optical elements, e.g. as lenses, prisms or the like in projection or illumination units or as beamforming elements in laser systems for microlithography. Apart from single crystals, it may also be possible to use crystalline fluoride materials which consist of a plurality of phases for producing optical components, as is described, for example, in GB 1 104 182.
Particularly when the crystal material is used in lens, illumination and laser system positions having a high radiation exposure, e.g. at energy densities greater than 50 mJ/cm2, there is the problem that the crystal material is damaged by the hard UV radiation used, which is, for example, produced by an excimer laser, if the crystal material has structural crystal defects (flaws). Such structural crystal defects are defects in the crystal structure which can be caused, inter alia, by incorporation of foreign atoms or foreign particles during crystallization. Under intensive UV irradiation, color centers which irreversibly reduce the transmission of the crystal with increasing radiation dose are formed at the flaws. In the case of high-purity crystals, the absorption coefficient tends to a saturation value with increasing radiation dose; this saturation value can serve as a characteristic measure of the laser resistance of the fluorite material, as is described in DE 10 2009 030 203 A1 of the applicant.
It is basically known that structural crystal defects can be influenced by an appropriately controlled growth and heat treatment process. Thus, for example, EP 0 939 147 A2 discloses a process in which a single crystal of calcium fluoride having improved optical properties is produced by carrying out a heat treatment process having a specific temperature profile on this.
In particular, it is also known that even tiny amounts of oxygen (in the ppm range), which are introduced as a result of the technology into the raw crystal mass in a growing apparatus, cause undesirable chemical secondary reactions in the melt of the crystal starting material or the raw crystal mass. Such secondary reactions lead to formation of crystal defects by incorporation of foreign atoms or ions into the crystal lattice and also to precipitation of microscopically small particles of metal oxides, e.g. CaO, PbO, or MgO, and thus lead to impairment of the optical properties of the crystal due to absorption or scattering.
The formation of such oxygen defect sites can be influenced by use of an appropriate protective gas atmosphere and use of scavengers. A scavenger is a purifying agent, i.e. a substance which serves to remove impurities from the raw crystal material. In the case of fluoride-containing crystals, an oxygen scavenger such as PbF2 is usually used for this purpose. It has been found that the addition of PbF2 brings about chemical reactions which lead to substantial removal of oxides, hydroxides or water. Here, for example, substances which are volatile even several hundred degrees Celsius below the melting point of calcium fluoride, e.g. PbO etc., are formed in a temperature range from 600° C. to 900° C.
Here, the impurities vaporize or sublime into the atmosphere surrounding the crystal and can there be removed from the plant (the growing furnace). Such a procedure is described, for example, in K. Th. Wilke, Kristallzüchtung, Deutscher Verlag der Wissenschaft, Berlin 1988, page 630 ff. Impurities in the crystal which, for example, deposit along small-angle grain boundaries can also be minimized by such purification processes.
However, it is known from, for example, DE 101 42 651 A1 that the use of a crystalline scavenger in the form of PbF2 can lead, from a laser resistance point of view, to not insignificant residues of foreign metal oxide (PbO) or fluoride (PbF2) in the crystal and these cannot be completely removed by distillation/sublimation.
To solve this problem, it has been proposed in, for example, the article “Czochralski growth of VV-grade CaF2 single crystals using ZnF2 additive as a scavenger”, J. Chryst. Growth 222, (2001), pages 243-248, that another metal fluoride, namely ZnF2, be used instead of PbF2 as scavenger. EP 0 869 203 A2, too, describes the production of a fluoride crystal, in which a scavenger in the form of ZnF2 is used in a plurality of process steps. However, ZnF2 and also the ZnO formed therefrom vaporize at higher temperatures than PbF2 and PbO. In addition, Zn residues also cause absorptions in the UV wavelength range from 150 nm to about 170 nm and influence the laser resistance at 193 nm.
EP 0 919 646 A1 describes the production of calcium fluoride crystals using scavengers, e.g. Teflon, and/or metal fluorides, e.g. PbF2, CoF2 and MnF2, but the above-described problems can also occur in the case of these scavenger materials. WO 01/025001 A1 also describes a process for producing fluoride crystals, in which use is made of a fluorinating material which is provided in the form of a solid, e.g. PbF2, NH4F, NH4F. HF or polytetrafluoroethylene (Teflon), or in the form of a gas, e.g. in the form of HF, F2 or NF3.
DE 101 42 651 A1 proposes using a reactive gas which comprises at least one substance containing reactive fluorine in addition to the crystalline scavenger in order to avoid problems with scavenger residues. This is said to be able to minimize the proportion of residues of the scavenger, e.g. PbO, in the crystal. Thus, for example, it is proposed that CF4, which reacts according to the equation 2PbO+CF4→2PbF2+CO2, be used as reactive gas. PbF2 reformed in this reaction can react again with oxygen-containing compounds or, should a reaction partner no longer be present, the fact that it has a somewhat lower boiling point than PbO and is therefore easier to remove can be exploited. The second compound formed, viz. CO2, is gaseous and can easily be removed under reduced pressure.
However, the process described in DE 101 42 651 A1 also has a number of disadvantages: since PbF2 has, at atmospheric pressure, a boiling point which is only about 200° C. below that of PbO, the PbF2 can remain and thus reduce the laser resistance. Although the vapor pressure of PbO (vapor pressure at 1500° C.-53.3 mbar (40 torr)) is a factor of about 10 lower than that of PbF2 (vapor pressure at 1500° C.-600 mbar (450 torr)), it cannot be assumed that all of the lead (Pb) can be removed from the plant, since lead compounds can deposit, for example, in graphite linings or on difficult-to-access places in the growing furnace. Furthermore, a secondary reaction in the form of PbO+CF4→PbF4+CO (redox reaction) can occur and carbon is incorporated in the form of graphite in the crystal (as black inclusions) as a result of the Boudouard equilibrium (CO2+C2CO), which likewise reduces the transmission and impairs other optical properties of the crystal.